One example of prior art analog signal interfaces is that with the signal inputs from the AC synchros which sense the actual angular position of various control surfaces on the aircraft, such as flaps, ailerons, rudder, etc. In contrast to the other types of input signals the synchro angle information resides contemporaneously in the relative magnitude and phasing between the line-to-line synchro stator voltages. As known, the rotor winding of each synchro sensor is energized with a known magnitude, reference AC voltage signal. The amplitude of the voltage signals induced in each of the synchro stator windings is dependent on the turns ratio between rotor and stator and on the synchro angle (.alpha.); by comparing the amplitude and relative phase of the voltages induced in each of two stator windings the value of .alpha. is uniquely determined.
In the prior art synchro signal interfaces the stator winding voltages (V.sub.X, V.sub.Y, V.sub.Z) are compared to provide two differential signals representative of the difference voltage between each of two stator windings with respect to the third stator winding selected as a reference. The differential synchro signals are sinusoids at the same frequency as the reference AC rotor signal, but with an amplitude which is modulated in dependence on the value of the synchro angle .alpha.. The pair of differential signals are converted by a precision Scott T network into a pair of output voltage signals whose amplitudes are proportional to the sine and cosine of the synchro angle value. As known, the Scott T network includes a pair of precision, cross coupled closed loop operational amplifiers, each of which provides one of the pair of output voltage signals. The sine and cosine signals are then processed in any one of a number of known methods to extract the relative magnitude and relative phasing information which defines the synchro angle value. One method includes phase shifting the sine and cosine signals by known, opposite phase shift values. The difference magnitude between the two phase shifted signals is representative of the synchro angle value, and is obtained by converting the phase shifted signal to a pulse width modulated (PWM) signal with a duty cycle in dependence on the difference signal magnitude. The PWM signal is converted into a digital word by counting the number of known frequency clock pulses within the ON portion of the PWM signal. The total count value is directly proportional to synchro angle value and may be read directly as a digital word to the DAU signal processor. An alternative method is to ratio the sine and cosine signals into a tangent or cotangent equivalent, which is converted to a digital signal equivalent and transformed by the DAU signal processor into the Arc function to generate the synchro angle value.
The prior art use of the Scott T circuitry, in addition to the downstream precision circuitry required to accurately preserve the magnitude and phase information in the two output signals, not only represents an additional dedicated interface within the overall analog interface, but a costly one at that. The circuitry is unique to the conditioning of the input synchro signal alone and has no applied use in the signal conditioning or conversion of any of the other types of analog input information. The present generation of digital flight data acquistion units (DFDAU) as defined by ARINC-717 requires a universal type signal conditioning interface which can accommodate all of the various types of input signal formats without the use of dedicated input channels. As such, any channel input must be capable of accepting any input signal format, i.e. DC, AC ratio, three wire resistance probes and synchros.